Hybrid electric vehicles (HEVs) utilize a combination of an internal combustion engine with an electric machine to provide the power needed to propel a vehicle. This arrangement provides improved fuel economy over conventional vehicles that possess only an internal combustion engine. An HEV may improve fuel economy by shutting down the engine during times that the engine operates inefficiently or is not otherwise needed to propel the vehicle. This may occur, for example, when the vehicle stops at traffic signals, stop signs, or when the electric machine is able to provide sufficient power to propel the vehicle without use of the engine. In these situations, the internal combustion engine is shutdown and the battery and electric machine provide power to propel the vehicle.
If additional power is requested or needed by the vehicle, the internal combustion engine is restarted using various methods. Such methods include a traditional starter motor or electric machine rotating the engine and firing the engine when the engine rotational speed exceeds a threshold value. These traditional engine start/restart methods may result in damage to the engine or transmission in the cases where the engine or transmission do not freely rotate, such as when the engine is hydrolocked, for example. As such, it is desirable to alter the engine restart strategy or avoid restarting the engine.